The current study was carried out to investigate the effects of Saccharomyces cerevisiae-fermented poultry by product meal (PBM) on growth performance, micromorphological, and immunological changes in common carp. Five experimental diets were prepared to include fermented PBM at 0, 5, 10, 15, and 20 % level in the diet of common carp (4.91±0.01 g). The fish were reared for 90 days on these diets. The obtained results revealed that yeast fermented PBM significantly changed the final body weight (FBW), weight gain (WG), specific growth rate (SGR), feed intake (FI), and feed conversion ratio (FCR) of the fish in a dose dependent manner (P<0.05). Fish fed 20 % fermented PBM showed the highest FBW, WG, SGR, FI, and lowest FCR. However, whole body composition did not vary significantly among fish fed different diets (P>0.05). Dietary yeast fermented PBM at 10 and 20 % level significantly increased the lipase, amylase, and protease activities than the other groups (P<0.05). The anterior, middle, and posterior intestinal villus length was significantly increased in fish fed fermented PBM at 15 and 20 % level when compared to the other groups (P<0.05). The number of goblet cells was significantly increased in the middle section of intestine in fish fed yeast fermented PBM at 20 % level, while in the posterior region of intestine the number of goblet cells was significantly increased in fish fed yeast fermented PBM at 15 and 20 % level (P<0.05). The histomorphology of intestine showed an increased length, branching and density of intestinal villi in fish fed yeast fermented PBM diets. Most of the measured blood parameters showed insignificant (P>0.05) differences except for Hb, RBCs, WBCs, total blood protein, and globulin which were significantly affected by the inclusion of yeast fermented PBM (P<0.05). Lysozyme activity was significantly increased in fish fed yeast fermented PBM at 5, 10, and 15 % level, while the phagocytic activity and phagocytic index were significantly increased at 20 % level when compared to the control groups (P<0.05). It is concluded from this study that inclusion of yeast fermented PBM in the diet of common carp at 15-20% level increased digestive enzyme activities, immune function and growth of the fish.
